In southern Lincoln County, Nevada, carbonate-rich dikes ranging in width from a few inches to about 40 ft intrude rhyolitic ignimbrites of the late Tertiary Kane Wash Formation. In the cryptocrystalline calcite matrix that forms 40 to 55 percent of the rock are embedded xenoliths of rhyolitic tuff and abundant crystals and crystal fragments of quartz and sanidine. Minor constituents are iron-oxide granules, andradite crystals, basalt fragments, and small glass particles, many of which are tubular. This intrusive rock lacks the trace-element composition and petrologic associations of accepted carbonatites. Carbon, oxygen, and strontium isotope ratios are not diagnostic. The chemical composition, the texture of the calcite, the forms of the glass particles, and the occurrence in ignimbrites that contain primary calcite indicate that the intrusive melt was formed by mixing of a silicate magma and melted limestone.